With the development of multimedia, flat panel displays (FDPs) are becoming more and more important. Accordingly, a variety of flat panel displays such as liquid crystal display (LCDs), plasma display panels (PDPs), field emission displays (FEDs), organic light emitting displays, and the like are put to practical use. Among them, the organic light emitting displays are drawing attention as next-generation displays because they have a fast response time of 1 ms or less and low power consumption but have no viewing angle issues because they emit light themselves.
Display devices are driven by using either a passive-matrix driving mode or an active-matrix driving mode using thin-film transistors. In the passive-matrix driving mode, is formed an anode and a cathode intersect each other and is driven by selecting a line, whereas, in the active-matrix driving mode, a thin-film transistor is connected to each pixel electrode, and each pixel is driven at a voltage maintained by the capacitance of a capacitor connected to the gate electrode of the thin film transistor.
It is very important for thin-film transistors to have durability and electrical reliability, as well as the basic characteristics such as mobility, leakage current, etc. The active layer in thin film transistors is usually formed of amorphous silicon or polycrystalline silicon. However, amorphous silicon is not electrically reliable despite its benefits like the simple film formation process and the low production cost. Polycrystalline silicon is hard to use over large-areas due to the high processing temperature, and does not provide uniformity for different methods of crystallization.
Because the active layer provides high mobility even if it is formed at low temperatures and the large variations in resistance with oxygen content make it very easy to obtain desired physical properties, an active layer made of oxide semiconductor is currently drawing a great deal of attention in thin-film transistor applications. Examples of oxide semiconductors that can be used as the active layer include zinc oxide (ZnO), indium zinc oxide (InZnO), or indium gallium zinc oxide (InGaZnO4). Thin-film transistors comprising an oxide semiconductor active layer may have various structures. Among them, coplanar and etch stopper structures are commonly used because of device characteristics.
FIG. 1 is a cross-sectional view showing a conventional coplanar thin-film transistor. FIG. 2 is a pattern diagram of atom diffusion. FIG. 3 is an image of a cross-section of a thin-film transistor. Referring to FIG. 1, a light blocking film 20 is positioned on a substrate 15, and a buffer layer 25 is positioned on the light blocking film 20. An active layer 30 of oxide semiconductor is formed on the buffer layer 25. A gate insulating film 35 and a gate electrode 40 are positioned on top of the active layer 30. An interlayer insulating film 45 is positioned on the gate electrode 40, and a source electrode 50a and a drain electrode 50b respectively are connected to the active layer 30, thereby forming a thin-film transistor 10. After the formation of the active layer 30, gate insulating film 35, and gate electrode 40, the thin-film transistor undergoes multiple subsequent thermal treatment processes. As shown in FIG. 2, once the subsequent thermal treatment processes are performed, atomic diffusion occurs in which hydrogen or oxygen atoms in the gate insulating film 35 diffuse into the active layer 30. Referring to FIG. 3, the A area in the active layer has a measured atomic ratio of In11Ga1Zn0.9O23.8, and the B area has a measured atomic ratio of In6.4Ga1Zn1.3O13.6, which means a high oxygen content at the interface between the active layer 30 and the gate insulating film 35.
Referring to FIG. 4, if the oxygen content at the interface between the active layer 30 and the gate insulating film 35 increases, this leads to an excess of unbound oxygen atoms. Oxygen is stable with two electrons, but each oxygen atom with an unpaired electron captures an electron moving through the channel in the active layer 30, thus degrading the device's characteristics.